On 02/26/10 11:42, Lutz Schumann wrote:
Idea:
- If the guest writes a block with 0's only, the block is freed again
- if someone reads this block again - it wil get the same 0's it would get
if the 0's would be written
- The checksum of a "all 0" block dan be hard coded for SHA1 / Flecher, so the comparison
for "is this a "0 only" block is easy.
With this in place, a host wishing to free thin provisioned zvol space can fill
the unused blocks wirth 0s easity with simple tools (e.g. dd if=/dev/zero
of=/MYFILE bs=1M; rm /MYFILE) and the space is freed again on the zvol side.
You've just described how ZFS behaves when compression is enabled -- a
block of zeros is compressed to a hole represented by an all-zeros block
pointer.
> Does anyone know why this is not incorporated into ZFS ?
It's in there. Turn on compression to use it.
- Bill
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